1. Field of the Invention
The present invention relates to a vacuum pressure control system for use in a semiconductor manufacturing apparatus or line.
2. Description of Related Art
In a CVD system in a semiconductor manufacturing apparatus or line, for instance, material gas which consists of elements which constitute a thin film material is supplied on wafers placed in a reaction chamber, while the inside of the reaction chamber is maintained under decompression, or vacuum. For example, in a CVD system shown in FIG. 14, the material gas is supplied on the wafers placed in the reaction chamber 111 which is a vacuum vessel through an inlet port 111 thereof. Simultaneously, the gas in the reaction chamber 110 is exhausted through an outlet port 112 of the reaction chamber 111 by suction of a vacuum pump 113. Thus, the inside of the reaction chamber 110 is maintained under vacuum.
At this time, it is necessary to maintain the vacuum pressure in there action chamber 110 constant. However, the constant value varies over a wide range of pressure from atmospheric pressure or a low vacuum near atmospheric pressure to a high vacuum according to various conditions. Then, in Japanese Patent No. 2,677,536, applicant of the present invention has disclosed a vacuum pressure control system capable of providing a constant vacuum over a wide range from a low vacuum near atmospheric pressure to a high vacuum.
FIG. 14 shows an example of the vacuum pressure control system. In such the vacuum pressure control system, the vacuum pressure in the reaction chamber 110 is measured by vacuum pressure sensors 114 and 115. In response to a difference between the measured pressure value and a desired vacuum pressure value given from the exterior, the control system controls the opening degree of a vacuum proportional opening and closing valve 116 provided with a poppet valve configuration.
The control system changes the conductance of an exhaust system from the reaction chamber 110 to the vacuum pump 113 in accordance with the opening degree of the opening and closing valve 116, and executes feedback-control on the vacuum pressure in the reaction chamber 110.
Thus, the control of the opening degree of the vacuum proportional valve 116 makes it possible to widely and surely change the conductance of the exhaust system. Accordingly, the vacuum pressure in the reaction chamber 110 can be maintained constant at a desired vacuum pressure value over a wide range from atmospheric pressure or a low vacuum near atmospheric pressure to a high vacuum.
In the conventional vacuum pressure control system, as mentioned above, the vacuum pressure sensors 114 and 115 measure the vacuum pressure in the reaction chamber 110, and the opening of the valve 116 is controlled in response to the difference between the measured vacuum pressure value and the desired vacuum pressure value, thereby changing the conductance of the exhaust system. However, the control system can not control the speed at which the vacuum pressure value in the reaction chamber 110 approaches the desired value (referred to as "vacuum pressure changing speed" hereinafter).
In the field of recent semiconductor manufacturing apparatus or line, it is required to prevent particles from flying up in the reaction chamber 110 in order to more improve the quality of a thin film formed on the wafer in the reaction chamber 110.
For that, when evacuation of the reaction chamber 110 is conducted so that the vacuum pressure value in the reaction chamber 110 which is under atmospheric pressure or a low vacuum near the atmospheric pressure reaches a desired value, the process of exhausting gas from the chamber 110 must be slowly conducted. However, the conventional vacuum pressure control system could not control the progress of exhausting gas from the chamber 110 and could not meet the above requirement.
The conventional vacuum pressure control system is therefore configured such that a bypass valve 117 having a fixed orifice is disposed in parallel to the vacuum proportional opening and closing valve 116, as shown in FIG. 14. In order to control the vacuum pressure in the chamber 110 to the desired vacuum pressure value, the bypass valve 117 is opened while the valve 116 is closed to reduce the vacuum pressure changing speed in the reaction chamber 110 so that the conductance of the exhaust system becomes a predetermined value.
The vacuum pressure changing speed which is reduced in the bypass valve 117 is dependent on only the conductance of the exhaust system when the velocity of gas flow (simply referred to as "gas velocity" hereinafter) which passes through the fixed orifice of the bypass valve 117 is in a sound speed region. On the other hand, the gas velocity shifts to a subsonic speed region when the vacuum pressure in the reaction chamber 110 is reduced to an absolute vacuum in proportion to the exhausted volume of gas therefrom. In this manner, when the gas velocity changes from sound speed to subsonic speed, the vacuum pressure changing speed in the reaction chamber 110 slows down in an inverse function.
When the vacuum pump 113 is actuated to start the exhaust of gas from the reaction chamber 110, of which the inside pressure is initially atmospheric pressure, the vacuum pressure changing speed in the reaction chamber 110 will quicken at a stroke if the fixed orifice of the bypass valve 117 which determines the conductance of the exhaust system is larger than necessary. This is not desirable at all from the viewpoint of preventing the particles from flying up in the reaction chamber 110.
Although a small fixed orifice which determines the conductance of the exhaust system is desirable from the viewpoint of preventing particles from flying up in the reaction chamber 110, it requires a considerable long time until the vacuum pressure in the reaction chamber 110 is regulated to the desired vacuum pressure, resulting in a problem that batch processing time in the chamber 110 is prolonged.
Such the problem would be resolved by using a plurality of bypass valves and needle valves in addition to the above mentioned bypass valve 117, all of which are disposed in parallel to the proportional valve 116. However, this configuration goes against the trend in recent years to reduce the size and cost of semiconductor manufacturing apparatus.